Re: [PATCH v2 -next] cpuset: fix warning when attaching tasks with offline CPUs

[Waiman Long <llong@redhat.com>]

On 7/14/25 10:33 PM, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
>
> A kernel warning was observed in the cpuset migration path:
>
>       WARNING: CPU: 3 PID: 123 at kernel/cgroup/cpuset.c:3130
>       cgroup_migrate_execute+0x8df/0xf30
>       Call Trace:
>        cgroup_transfer_tasks+0x2f3/0x3b0
>        cpuset_migrate_tasks_workfn+0x146/0x3b0
>        process_one_work+0x5ba/0xda0
>        worker_thread+0x788/0x1220
>
> The issue can be reliably reproduced with:
>
>       # Setup test cpuset
>       mkdir /sys/fs/cgroup/cpuset/test
>       echo 2-3 > /sys/fs/cgroup/cpuset/test/cpuset.cpus
>       echo 0 > /sys/fs/cgroup/cpuset/test/cpuset.mems
>
>       # Start test process
>       sleep 100 &
>       pid=$!
>       echo $pid > /sys/fs/cgroup/cpuset/test/cgroup.procs
>       taskset -p 0xC $pid  # Bind to CPUs 2-3
>
>       # Take CPUs offline
>       echo 0 > /sys/devices/system/cpu/cpu3/online
>       echo 0 > /sys/devices/system/cpu/cpu2/online
>
> Root cause analysis:
> When tasks are migrated to top_cpuset due to CPUs going offline,
> cpuset_attach_task() sets the CPU affinity using cpus_attach which
> is initialized from cpu_possible_mask. This mask may include offline
> CPUs. When __set_cpus_allowed_ptr() computes the intersection between:
> 1. cpus_attach (possible CPUs, may include offline)
> 2. p->user_cpus_ptr (original user-set mask)
> The resulting new_mask may contain only offline CPUs, causing the
> operation to fail.
>
> To resolve this issue, if the call to set_cpus_allowed_ptr fails, retry
> using the intersection of cpus_attach and cpu_active_mask.
>
> Fixes: da019032819a ("sched: Enforce user requested affinity")
> Suggested-by: Waiman Long <llong@redhat.com>
> Reported-by: Yang Lijin <yanglijin@huawei.com>
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>   kernel/cgroup/cpuset.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)

Thanking further about this problem, the cpuset patch that I proposed is 
just a bandage. It is better to fix the problem at its origin in 
kernel/sched/core.c. I have posted a new patch to do that.

Cheers,
Longman

>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index f74d04429a29..2cf788a8982a 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3114,6 +3114,10 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>   static cpumask_var_t cpus_attach;
>   static nodemask_t cpuset_attach_nodemask_to;
>   
> +/*
> + * Note that tasks in the top cpuset won't get update to their cpumasks when
> + * a hotplug event happens. So we include offline CPUs as well.
> + */
>   static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>   {
>   	lockdep_assert_held(&cpuset_mutex);
> @@ -3127,7 +3131,16 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>   	 * can_attach beforehand should guarantee that this doesn't
>   	 * fail.  TODO: have a better way to handle failure here
>   	 */
> -	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
> +	if (unlikely(set_cpus_allowed_ptr(task, cpus_attach))) {
> +		/*
> +		 * Since offline CPUs are included for top_cpuset,
> +		 * set_cpus_allowed_ptr() can fail if user_cpus_ptr contains
> +		 * only offline CPUs. Take out the offline CPUs and retry.
> +		 */
> +		if (cs == &top_cpuset)
> +			cpumask_and(cpus_attach, cpus_attach, cpu_active_mask);
> +		WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
> +	}
>   
>   	cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
>   	cpuset1_update_task_spread_flags(cs, task);